1. Technical Field
The present invention relates to magnetic recording media capable of recording large capacity of information and, more particularly, it relates to a magnetic recording medium suitable to high-density magnetic recording.
2. Description of the Related Art
A demand for increasing the capacity in magnetic storage apparatus typically represented by a magnetic disk drive has been increased more and more. In order to cope with the demand, magnetic heads at high sensitivity or recording media capable of obtaining high signal output-to-noise ratio: S/Nd have been desired.
Generally, a recording medium comprises a first underlayer referred to as a seed layer, a second underlayer of a body-centered cubic structure comprising a chromium alloy, a magnetic layer, and a protection film comprising carbon as a main ingredient, which are formed on a substrate. For the magnetic layer an alloy having a hexagonal close-pack structure comprising cobalt as a main ingredient is used.
To improve S/Nd, it is effective to provide a magnetic layer with crystallographic orientation in which (11.0) face, or (10.0) face are substantially in parallel with the surface of a substrate and direct the c-axis of the hexagonal close-pack structure as an axis of easy magnetization within the film plane. It has been known that the crystallographic orientation of the magnetic layer can be controlled by a seed layer and the orientation can be attained by using tantalum or B2 structure NiAl alloy for the seed layer. It has also been known that magnetic characteristics in the circumferential direction can be improved by applying mechanical texturing to the surface of a substrate to introduce magnetic anisotropy in the circumferential direction.
To improve S/Nd, it is effective to adopt a multi-layered constitution for the magnetic layer, refine the crystal grain size and reduce Brt as a product of a residual magnetic flux density (Br) and a film thickness (t) of the magnetic layer. That is, a magnetic recording medium has been proposed in which an underlayer is formed on a substrate, and stacked magnetic films constituted with magnetic layers comprising two layers of different compositions in contact with each other are disposed thereon as a multi-layered constitution by way of a non-magnetic layer such as made of ruthenium.
In addition, another magnetic recording medium including an underlayer and a magnetic recording layer formed thereover on a substrate has been proposed in which the magnetic recording layer has a multi-layered structure separated vertically by an intermediate layer, the intermediate layer is formed of one of materials selected from the group consisting of Ru, Rh, Ir and an alloy thereof selected from a range of 0.2 nm to 0.4 nm and 1.0 nm to 17 nm and directions of magnetization for the magnetic recording layers separated vertically by the intermediate layer are in parallel with each other. The use of such a magnetic recording medium having a magnetic recording layer of a multi-layered structure achieves thermal stability and reduced noise together with maintenance of magnetic characteristics.
When crystal particles used for the magnetic recording layer are refined extremely, or Brt, is reduced greatly, thermal stability is deteriorated and, accordingly, there is a limit for the reduction of noise. In recent years, an anti-ferromagnetic coupled (AFC) medium to be described later has been proposed as a technique of making the thermal stability and the noise reduction compatible with each other. This medium has a dual-layered structure in which two magnetic layers are anti-ferromagnetically coupled by way of an Ru intermediate layer, which can reduce Brt more while keeping the magnetic film thickness compared with a medium comprising a single-layered magnetic layer. This enables reduced medium noise together with maintenance of the thermal stability.
U.S. Patent Application Publication No. 2002/98390A1, proposes a magnetic recording medium having a substrate and a magnetic recording layer on the substrate, in which the magnetic recording layer comprises an the AFC layer, a ferromagnetic layer and a non-magnetic spacer layer for separating the AFC layer and the ferromagnetic layer, the AFC layer comprises a first ferromagnetic layer, a second ferromagnetic layer, and a layer present between the first and the second ferromagnetic layer for anti-ferromagnetically coupling them, the anti-ferromagnetically coupling layer of the AFC layer has a film thickness and a composition for providing anti-ferromagnetic exchange coupling between the first and the second ferromagnetic layer, and the non-ferromagnetic spacer formed between the second magnetic layer of the AFC layer and the ferromagnetic layer has a film thickness and a composition providing no exchange coupling between the second magnetic layer of the AFC layer and the ferromagnetic layer.